In recent years, chargeable and dischargeable secondary batteries are widely used as energy sources of wireless mobile equipment. Of these, lithium secondary batteries are generally used due to advantages such as high energy density, discharge voltage and power stability.
Lithium secondary batteries use metal oxide such as LiCoO2 as a cathode material and carbon as an anode material and are fabricated by inserting a polyolefin-based porous membrane between an anode and a cathode and swelling a non-aqueous electrolyte containing a lithium salt such as LiPF6. LiCoO2 is commonly used as a cathode material for lithium secondary batteries. LiCoO2 has several disadvantages of being relatively expensive, having low charge/discharge capacity of about 150 mAh/g and unstable crystal structure at a voltage of 4.3 V or higher and the risk of reacting with an electrolyte to cause combustion. Furthermore, LiCoO2 is disadvantageous in that it undergoes great variation in physical properties depending upon variation in parameters in the preparation process thereof. In particular, cycle properties and high-temperature storage properties at high electric potential may be greatly varied depending on partial variations of process parameters.
In this regard, methods to make batteries containing LiCoO2 operate at high electric potential, such as coating the outer surface of LiCoO2 with a metal (such as aluminum), thermally treating LiCoO2, or mixing LiCoO2 with other materials, have been suggested. Secondary batteries comprising such a cathode material exhibit low stability at high electric potential or have a limitation of application to mass-production.
In recent years, secondary batteries receive great attention as power sources of electric vehicles (EVs), hydride electric vehicles (HEV) or the like which are suggested as alternatives to conventional gasoline vehicles, diesel vehicles or the like using fossil fuels to solve air pollution caused thereby. Use of secondary batteries is expected to further increase and the above problems and problems associated with stability of batteries and high-temperature storage properties at high electric potentials arise.
In an attempt to solve the problems of LiCoO2, methods using a mixture of two or more different lithium transition metal oxides as a cathode material were suggested. These methods solve the drawbacks of a cathode material in which the respective lithium transition metal oxide is used singly.
However, conventional mixture-type cathode materials have a limitation of the difficulty of obtaining superior synergetic effects to the case of simple combination of two ingredients.
Meanwhile, a great deal of methods to solve battery stability has been suggested. However, a clear method for solving combustion of batteries caused by compulsory internal short circuit (in particular, customer-abuse) has not been suggested to date.
In recent years, coating a polyolefin-based membrane with an inorganic substance such as calcium carbonate or silica to prevent internal short circuit caused by dendrite growth in batteries was reported in U.S. Pat. No. 6,432,586. However, use of common inorganic particles has no mechanism to prevent internal short circuits caused by external impact, having no great effects on substantially improving stability. In addition, parameters such as thickness, pore size and porosity of mentioned inorganic layers are not suitably determined and the inorganic particles have no capability to transfer lithium. Accordingly, considerable deterioration in battery capability is inevitable.